1. Field of the Invention
The present invention relates to an immersion solution for a microscope, which is used for an optical apparatus.
2. Description of the Related Art
In optical apparatuses such as a microscope, various types of immersion oils are used as an immersion solution. An immersion solution is a liquid used for optical characteristics improvement by charging the immersion solution into a space in which, e.g., air is present between optical components of an optical apparatus or between an optical component and an object of the optical apparatus. For example, in an optical apparatus for observing an specimen placed close to a lens, such as a microscope, charge of an oil having a large refractive index between an objective lens and a specimen as an immersion oil enables not only a decrease in optical aberrations but also an increase in numerical aperture of the objective lens to raise the magnification of the objective lens. For example, Japanese Patent Application Laid-Open Publication No. 2002-53839 discloses an immersion oil with a refractive index increased by mixing antimony halide such as antimony tribromide into a liquid organic compound such as diethylene glycol.
Meanwhile, fluorescence imaging in which a specimen is illuminated with excitation light having a certain wavelength to image/measure feeble fluorescence generated by the specimen is performed using, e.g., fluorescence microscopes. In fluorescence imaging, a bright optical system having a large numerical aperture is used to image feeble fluorescence; however, it is necessary that an immersion oil to be used in a fluorescence microscope including an optical system having a large numerical aperture have a high refractive index.
Furthermore, in fluorescence imaging, fluorescence generated by an immersion oil (auto fluorescence) acts as background noise in an fluorescence image observed; however, the aforementioned high refractive index immersion oil is not one manufactured for use in fluorescence measurement, and thus, it is difficult to perform fluorescence imaging using such high refractive index immersion oil. Thus, in Japanese Patent Application Laid-Open Publication No. 2008-134517, the present applicant discloses that an immersion oil with small auto fluorescence can be obtained by removing impurities from a high refractive index immersion oil obtained by adding, e.g., sulfur in diiodomethane.
Also, in time-plus imaging in which a cell is imaged for several days with a temperature of the cell maintained at 37° C., a problem of deterioration in characteristics of an immersion oil resulting from a decomposition reaction sometimes occurs. Thus, there has been a demand for a more stable, that is, persistent and non-volatile immersion solution.
Meanwhile, ionic liquids, which are also referred to as ambient temperature molten salts or room temperature molten salts, each have a low melting point in spite of being salts, and thus, the ionic liquids melt even at around room temperature, and exhibit high conductivity. Ionic liquids have drawn attention as electrolytes because of their characteristics such as non-volatility and flame resistance. Also, Japanese Patent Application Laid-Open Publication No. 2010-14808 discloses a solution containing an ionic liquid material is used as a lens.